Ian
Notorious member
The displaced lead thing is super-important for design considerations. It takes force to swage a slightly oversized cast bullet through the throat, but we have to have slight interference in order to assure that the bullet obturates the bore. Think about how much force it will take to cram the bullet all the way into the rifling compare that to how much (or little) force it will take to bend the base of the bullet to one side in the unsupported chamber neck area. My approach is make going straight up the middle the easiest course for the bullet, or "path of least resistance". Creating a purposeful path for bullet metal to displace is sort of why we don't typically have really wide (long) driving bands. I also believe that easing the metal displacement is why bullets having very square lube grooves tend to shoot very well, and why Loverin bullets also tend to shoot well up to the velocity limit of their bearing surface.
Another thing to think about is throat wear. I found it interesting to note that after making pound casts from a variety of rifles that the throat wear tends to follow a trend. I'm far from the first person to discover this, but it was good to verify and learn what wear angles those actually are. So I designed a bullet around that and compared it to another bullet which also works in a lot of different .30-caliber rifles quite well, without needing the nose to be engraved in the rifling in order to ensure a straight start in the bore. Being able to "float the nose" is a requirement for self-loading rifles and is an extra bit of insurance against pulling a bullet when unloading a hunting rifle at the end of the day or to cross a fence. There is a lot more "technique" to getting a floating-nose cartridge to shoot straight with cast bullets than there is with one which has most of the bullet shoved into the rifling with a glove-fit, but it works just as well if you get everything just right.
The MP 312-180 Silhouette designed by 45 2.1 and the AM 31-188G designed by yours truly take all the above design considerations into account. Recovered MP SIL bullets reveal how the tapered front band folds back into the lube groove during firing, like closing an umbrella, and it does so in a very concentric manner. This allows the bullet to be sized up to .313" for a snug loaded fit in the chamber neck and still shoot well in a barrel with a .308" or even smaller groove diameter. The square band behind the lube groove self-centers very nicely and displaces metal in a very concentric manner compared to triangular lube grooves (don't ask me why that is, but recovered bullets speak for themselves). Selecting an alloy which fits the velocity and pressure as well as pressure curve (how hard/fast that front band slams into the taper of the throat) is paramount to achieving success with the .30 SIL and other designs. Low antimony alloy, heat treated, with even lower tin content works best for me with these designs due to the way the metal likes to shear and flow.
Another thing to think about is throat wear. I found it interesting to note that after making pound casts from a variety of rifles that the throat wear tends to follow a trend. I'm far from the first person to discover this, but it was good to verify and learn what wear angles those actually are. So I designed a bullet around that and compared it to another bullet which also works in a lot of different .30-caliber rifles quite well, without needing the nose to be engraved in the rifling in order to ensure a straight start in the bore. Being able to "float the nose" is a requirement for self-loading rifles and is an extra bit of insurance against pulling a bullet when unloading a hunting rifle at the end of the day or to cross a fence. There is a lot more "technique" to getting a floating-nose cartridge to shoot straight with cast bullets than there is with one which has most of the bullet shoved into the rifling with a glove-fit, but it works just as well if you get everything just right.
The MP 312-180 Silhouette designed by 45 2.1 and the AM 31-188G designed by yours truly take all the above design considerations into account. Recovered MP SIL bullets reveal how the tapered front band folds back into the lube groove during firing, like closing an umbrella, and it does so in a very concentric manner. This allows the bullet to be sized up to .313" for a snug loaded fit in the chamber neck and still shoot well in a barrel with a .308" or even smaller groove diameter. The square band behind the lube groove self-centers very nicely and displaces metal in a very concentric manner compared to triangular lube grooves (don't ask me why that is, but recovered bullets speak for themselves). Selecting an alloy which fits the velocity and pressure as well as pressure curve (how hard/fast that front band slams into the taper of the throat) is paramount to achieving success with the .30 SIL and other designs. Low antimony alloy, heat treated, with even lower tin content works best for me with these designs due to the way the metal likes to shear and flow.
